Fragmentation, amalgamation and twisted Hilbert spaces Daniel - PowerPoint PPT Presentation

Fragmentation, amalgamation and twisted Hilbert spaces Daniel Morales Gonz´ alez Departamento de Matem´ aticas Universidad de Extremadura September 12, 2019 This work was supported by project MTM2016-76958-C2-1-P

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation 1 Palais’ problem and twisted Hilbert spaces 2 Complex interpolation and derivations 3 Fragmentation and amalgamation Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation 1 Palais’ problem and twisted Hilbert spaces 2 Complex interpolation and derivations 3 Fragmentation and amalgamation Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Palais’ problem Let X be a Banach space, and let Y be a closed subspace of X . Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Palais’ problem Let X be a Banach space, and let Y be a closed subspace of X . Problem (Palais’) If Y and X/Y are isomorphic to a Hilbert space, has X to be isomorphic to a Hilbert space? Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Palais’ problem Let X be a Banach space, and let Y be a closed subspace of X . Problem (Palais’) If Y and X/Y are isomorphic to a Hilbert space, has X to be isomorphic to a Hilbert space? The theory of twisted Hilbert spaces grew from this problem. Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Twisted Hilbert spaces Definition A twisted Hilbert space is a Banach space X with a subspace Y isomorphic to a Hilbert space such that the corresponding quotient X/Y is also isomorphic to a Hilbert space. Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Twisted Hilbert spaces Definition A twisted Hilbert space is a Banach space X with a subspace Y isomorphic to a Hilbert space such that the corresponding quotient X/Y is also isomorphic to a Hilbert space. In homological terms, it is the space in the middle of a short exact sequence 0 − → H − → X − → H − → 0 . Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Twisted Hilbert spaces Definition A twisted Hilbert space is a Banach space X with a subspace Y isomorphic to a Hilbert space such that the corresponding quotient X/Y is also isomorphic to a Hilbert space. In homological terms, it is the space in the middle of a short exact sequence 0 − → H − → X − → H − → 0 . The first twisted Hilbert space that one can see is the proper Hilbert space Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Twisted Hilbert spaces We say that a twisted Hilbert is trivial when the exact sequence splits, or equivalently Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Twisted Hilbert spaces We say that a twisted Hilbert is trivial when the exact sequence splits, or equivalently the space in the middle is the direct sum of the subspace and the quotient, Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Twisted Hilbert spaces We say that a twisted Hilbert is trivial when the exact sequence splits, or equivalently the space in the middle is the direct sum of the subspace and the quotient, the subspace is complemented, Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Twisted Hilbert spaces We say that a twisted Hilbert is trivial when the exact sequence splits, or equivalently the space in the middle is the direct sum of the subspace and the quotient, the subspace is complemented, there exists a continuous proyection from the space to the subspace. Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Twisted Hilbert spaces We say that a twisted Hilbert is trivial when the exact sequence splits, or equivalently the space in the middle is the direct sum of the subspace and the quotient, the subspace is complemented, there exists a continuous proyection from the space to the subspace. The first non-trivial twisted Hilbert was obtained by Enflo, Lindenstrauss and Pisier, giving a negative answer to Palais’ problem. Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation ELP space This twisted Hilbert of Enflo, Lindenstrauss and Pisier, (ELP) has the form ℓ 2 ( F n ), where F n are finite-dimensional Banach spaces. Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation ELP space This twisted Hilbert of Enflo, Lindenstrauss and Pisier, (ELP) has the form ℓ 2 ( F n ), where F n are finite-dimensional Banach spaces.Precisely, they constructed these exact sequences → ELP n − P n → ℓ n 2 → ℓ n 0 − − 2 − → 0 2 in such a way that lim n →∞ � P n � = ∞ . Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation ELP space This twisted Hilbert of Enflo, Lindenstrauss and Pisier, (ELP) has the form ℓ 2 ( F n ), where F n are finite-dimensional Banach spaces.Precisely, they constructed these exact sequences → ELP n − P n → ℓ n 2 → ℓ n 0 − − 2 − → 0 2 in such a way that lim n →∞ � P n � = ∞ . So pasting all with the ℓ 2 norm it results → ℓ 2 ( ℓ n 2 → ℓ 2 ( ELP n ) − → ℓ 2 ( ℓ n 0 − 2 ) − 2 ) − → 0 , and cannot exists a continouos proyection to the subspace. Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation 1 Palais’ problem and twisted Hilbert spaces 2 Complex interpolation and derivations 3 Fragmentation and amalgamation Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Complex interpolation Other method to construct twisted Hilbert spaces is by complex interpolation. Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Complex interpolation Other method to construct twisted Hilbert spaces is by complex interpolation. Let S be the open strip { z ∈ C : 0 < Re ( z ) < 1 } and let ¯ S its closure. Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Complex interpolation Other method to construct twisted Hilbert spaces is by complex interpolation. Let S be the open strip { z ∈ C : 0 < Re ( z ) < 1 } and let ¯ S its closure. Given an admissible pair ( X 0 , X 1 ) of complex Banach spaces, let Σ = X 0 + X 1 endowed with the norm � x � = inf {� x 0 � 0 + � x 1 � 1 : x = x 0 + x 1 } . Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Complex interpolation We denote F ( X 0 , X 1 ) to the space of functions f : ¯ S → Σ satisfying these conditions: Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Complex interpolation We denote F ( X 0 , X 1 ) to the space of functions f : ¯ S → Σ satisfying these conditions: f is � · � Σ -bounded and � · � Σ -continuous on ¯ S , Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Complex interpolation We denote F ( X 0 , X 1 ) to the space of functions f : ¯ S → Σ satisfying these conditions: f is � · � Σ -bounded and � · � Σ -continuous on ¯ S , f is � · � Σ -analytic on S , Daniel Morales Gonz´ alez

Palais’ problem and twisted Hilbert spaces Complex interpolation and derivations Fragmentation and amalgamation Complex interpolation We denote F ( X 0 , X 1 ) to the space of functions f : ¯ S → Σ satisfying these conditions: f is � · � Σ -bounded and � · � Σ -continuous on ¯ S , f is � · � Σ -analytic on S , f ( it + j ) ∈ X j , ( j = 0 , 1) and the map t ∈ R �→ f ( it + j ) is bounded and continuous. Daniel Morales Gonz´ alez